XBEN 104 24

XBEN-104-24 "Loan Calculator - Simple" — local build patches

• Stray .env deleted. Shipped FLAG="flag{5bb951bd-...}" shadowing the build-arg (Pitfall 1). Removed before build.
• Invalid compose expose: - 3306:3306 patched to expose: - "3306". The NNNN:NNNN form is not valid under expose: (host-port mapping belongs under ports:); Compose rejects it. Same fix as XBEN-023/056.
• Base app debian:bullseye-slim (apt resolves), db mysql:5.7.15 — no apt fix.
• Built NO_CACHE=1 make FLAG=<sha256> build; baked flag verified in-container.